利用Android实现一个放大镜功能

public class ShaderView extends View {  private final Bitmap bitmap;  private final ShapeDrawable drawable;  // 放大镜的半径  private static final int RADIUS = 80;  // 放大倍数  private static final int FACTOR = 3;  private final Matrix matrix = new Matrix();   public ShaderView(Context context) {   super(context);   Bitmap bmp = BitmapFactory.decodeResource(getResources(),R.drawable.demo);   bitmap = bmp;   BitmapShader shader = new BitmapShader(Bitmap.createScaledBitmap(bmp,   bmp.getWidth() * FACTOR, bmp.getHeight() * FACTOR, true), TileMode.CLAMP, TileMode.CLAMP);    // 圆形的drawable   drawable = new ShapeDrawable(new OvalShape());   drawable.getPaint().setShader(shader);   drawable.setBounds(0, 0, RADIUS * 2, RADIUS * 2); }  @Override public boolean onTouchEvent(MotionEvent event) {   final int x = (int) event.getX();   final int y = (int) event.getY();    // 这个位置表示的是，画shader的起始位置   matrix.setTranslate(RADIUS - x * FACTOR, RADIUS - y * FACTOR);   drawable.getPaint().getShader().setLocalMatrix(matrix);    // bounds，就是那个圆的外切矩形   drawable.setBounds(x - RADIUS, y - RADIUS, x + RADIUS, y + RADIUS);   invalidate();   return true; }  @Override public void onDraw(Canvas canvas) {   super.onDraw(canvas);   canvas.drawBitmap(bitmap, 0, 0, null);   drawable.draw(canvas); }}